SPLITTING STARS

A double's separation is the apparent distance of the companion, or secondary star, from the brighter star, the primary. Albireo (Beta Cygni), a beautiful double with colors described as gold and sapphire, has a separation of 34 arcseconds.

SEEING DOUBLE

You do not need a large telescope to observe double stars. A 3 inch (75 mm) refractor can produce excellent results, since its narrow field renders high-contrast, pinpoint star images. A 6 inch (150 mm) scope, though, is better for resolving close doubles.

POSITION ANGLE

The position of the secondary star with respect to the primary is called the position angle, or
PA. The primary star is designated component A, and the secondary, component B.
Other components, if any, are listed as C, D, and so forth.

MAGNITUDE AND COLOR DIFFERENCES

Double stars of different magnitudes and small separa-tions can be both a joy and a challenge to observe. Tau Eridani's 5.7 arcsecond separation would not usually be a problem for, say, a 6 inch (150 mm) telescope to split. But in this case, the primary's magnitude of 4.0 can overwhelm the magnitude 10.0 secondary, particularly in pool-seeing. Still, it is not impossible, and seeing a bright star next to a faint one with just a hairbreadth between them can be breathtaking.

DOUBLE DEDICATION

The first double star ever recorded—Mizar in the Big Dipper— was discovered accidentally in 1650 by the Italian astronomer Giovanni Battista Riccioli. Subsequent discoveries by other astronomers were also accidental. By 1779, enough observations had been compiled to inspire the indefatigable William Herschel (1738-1822) to begin a systematic search for these stellar curiosities.